A rapid and sensitive reversed phase liquid chromatography- tandem mass spectrometry (LC-MS-MS) method is developed for the determination of doxazosin in canine plasma. The samples are prepared by precipitation of proteins using a mixture of methanol and acetonitrile, followed by freezing and evaporation of the organic solvent. The remaining dry residue is redissolved in mobile phase and analyzed by LC-MS-MS with positive electrospray ionization using the selected reactions monitoring mode. An XTerra MS C(18) column, a mobile phase composed of acetonitrile and 2mM ammonium acetate with gradient elution, and a flow rate of 400 microL/min are employed. The elution times for prazosin (internal standard) and doxazosin are approximately 8 and 10 min, respectively. Calibration curves are linear in the 1-20 ng/mL concentration range. Limits of detection and quantification are 0.4 ng/mL and 1.2 ng/mL, respectively. Recovery is higher than 94%. Intra- and inter-day relative standard deviations are below 7% and 8%, respectively. The method is applied for the determination of doxazosin plasma levels following a single administration of doxazosin base and doxazosin mesylate tablets (2 mg dose) to dogs in the fed state. The results indicate possible superiority of the mesylate salt on the plasma input rates of doxazosin.